Recently, wireless local area networks (LANs) have been developed as an enhanced replacement for wired LANs. In a wireless LAN a plurality of mobile network stations (e.g., personal computers, telecommunication devices, etc.) are present that are capable of wireless communication. As compared to wired LANs, data-communication in a wireless LAN can be more versatile, due to the flexibility of the arrangement of network stations in the area covered by the LAN, and due to the absence of cabling connections.
Wireless LANs are generally implemented according to the standard as defined by the ISO/IEC 8802-11 international standard (IEEE 802.11). IEEE 802.11 describes a standard for wireless LAN systems that will operate in the 2.4-2.5 GHz ISM (industrial, scientific and medical) band. This ISM band is available worldwide and allows unlicensed operation for spread spectrum systems. The 2,400-2,483.5 MHz band has been allocated for both the US and Europe. The IEEE 802.11 standard focuses on the MAC (medium access control) and PHY (physical layer) protocols for access point based networks and ad-hoc networks.
In access point based wireless networks, the stations within a group or cell can communicate only directly to the access point. This access point forwards messages to the destination station within the same cell or through the wired distribution system to another access point, from which such messages arrive finally at the destination station.
The 802.11 standard supports three PHY protocols: DSSS (direct sequence spread spectrum), FHSS (frequency hopping spread spectrum), and infrared with PPM (pulse position modulation). All these three PHYs all provide bit rates of 1 and 2 Mbit/s. Furthermore, IEEE 802.11 includes extensions 11a, 11b, and 11g which allow for additional higher bit rates: Extension 11b provides bit rates 5.5 and 11 Mbit/s as well as the basic DSSS bit rates of 1 and 2 Mbit/s within the same 2.4-2.5 GHz ISM band. Extension 11a provides a high bit rate OFDM (Orthogonal Frequency Division Multiplexing modulation) PHY standard providing bit rates in the range of 6 to 54 Mbit/s in the 5 GHz band. Extension 11g provides a high bit rates up to 54 Mbit/s using OFDM in the 2.4-2.5 GHz ISM band.
The IEEE 802.11 basic MAC protocol allows interoperability between compatible PHYs through the use of the CSMA/CA (carrier sense multiple access with collision avoidance) protocol and a random back-off time following a busy medium condition. The IEEE 802.11 CSMA/CA protocol is designed to reduce the collision probability between multiple stations accessing the medium at the same time. Therefore, a random back-off arrangement is used to resolve medium contention conflicts. In addition, the IEEE 802.11 MAC protocol defines special functional behavior for fragmentation of packets, medium reservation via RTS/CTS (request-to-send/clear-to-send) polling interaction and point coordination (for time-bounded services).
Moreover, the IEEE 802.11 MAC protocol defines Beacon frames sent at regular intervals by the access point to allow stations to monitor the presence of the access point. The IEEE 802.11 MAC protocol also gives a set of management frames including Probe Request frames which are sent by a station and are followed by Probe-Response frames sent by an available access point, to allow a station to scan actively if there is an access point operating on a certain channel frequency and to show to the station what parameter settings this access point is using.
Wireless personal digital assistants (WPDA) and “smart” cellular telephones are the most popular devices known to provide “mobile Internet” access for the mass consumer market.